drm/i915/guc/slpc: Allow SLPC to use efficient frequency

Host Turbo operates at efficient frequency when GT is not idle unless
the user or workload has forced it to a higher level. Replicate the same
behavior in SLPC by allowing the algorithm to use efficient frequency.
We had disabled it during boot due to concerns that it might break
kernel ABI for min frequency. However, this is not the case since
SLPC will still abide by the (min,max) range limits.

With this change, min freq will be at efficient frequency level at init
instead of fused min (RPn). If user chooses to reduce min freq below the
efficient freq, we will turn off usage of efficient frequency and honor
the user request. When a higher value is written, it will get toggled
back again.

The patch also corrects the register which needs to be read for obtaining
the correct efficient frequency for Gen9+.

We see much better perf numbers with benchmarks like glmark2 with
efficient frequency usage enabled as expected.

v2: Address review comments (Rodrigo)
v3: with efficient frequency being dynamic, it is possible that the req
frequency may go beyond max freq. This will cause SLPC selftests to fail.
Add a FIXME there to start the test with [RPn, RP0] instead and restore
it afterwards.

BugLink: https://gitlab.freedesktop.org/drm/intel/-/issues/5468



Cc: Rodrigo Vivi <rodrigo.vivi@intel.com>
Signed-off-by: Vinay Belgaumkar <vinay.belgaumkar@intel.com>
Signed-off-by: Rodrigo Vivi <rodrigo.vivi@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20220820010832.15350-1-vinay.belgaumkar@intel.com